Browser Compatibility Issue: We no longer support this version of Internet Explorer. For optimal site performance we recommend you update your browser to the latest version.Update Microsoft Internet Explorer

LTspice: Using Time-Dependent Exponential Sources to Model Transients

When working with Surge Stoppers or Hot Swap Controllers, occasionally there is a need to simulate a circuit's behavior with a specified voltage or current transient. These transients are usually modeled using a double exponential waveform characterized by a peak voltage, a rise time (usually 10%–90%), a fall time to 50% of the peak voltage and a series resistance.

LTspice® features a double exponential function (EXP) that is ideal for modeling transients via a voltage source. However, it is not as simple as filling in the parameter with tRISE, tFALL and VPEAK. Instead, the EXP function uses standard parameters: VINITIAL, VPULSED, Rise Delay, Fall Delay, Raise Tau, and Fall Tau.

For waveforms where tFALL : tRISE> 50 : 1 where tFALL is defined from 100%–50% and tRISE is defined from 10%–90%, you can use the following conversions for the EXP function parameters, and under the voltage source’s parasitic properties, enter the appropriate series resistance or add it to the schematic as a separate component:

The waveforms below show the results of the above EXP voltage source with an open circuit, VGEN, and clamped with a TVS clamp, VIN. Also shown is the instantaneous power dissipation (Alt + left-click) of the TVS. The second waveform highlights the rise time portion.

To simulate repeated bursts of transients as in Electrical Fast Transient, LTspice provides an extended syntax for the EXP function that is undocumented and not available in the standard component editor.

EXP(V1 V2 Td1 Tau1 Td2 Tau2 Tpulse Npulse Tburst)

Where Tpulse is the pulse period, Npulse is the number of pulses per burst and Tburst is the repeat burst period. To add these to your exisitng EXP fuction, edit the EXP text string directly in your schematic by right-clicking it.

The following example shows a burst of 75 transients with a period of 10μs.

Analog Devices Uses Cookies for Enhanced Online Performance

Some cookies are required for secure log-ins but others are optional for functional activities. Our data collection is used to improve our products and services. We recommend you accept our cookies to ensure you’re receiving the best performance and functionality our site can provide. For additional information you may view the cookie details. Read more about our privacy policy.

These are cookies that are required for the operation of analog.com or specific functionality offered. They either serve the sole purpose of carrying out network transmissions or are strictly necessary to provide an online service explicitly requested by you.

Analytics/Performance Cookies:

These cookies allow us to carry out web analytics or other forms of audience measuring such as recognizing and counting the number of visitors and seeing how visitors move around our website. This helps us to improve the way the website works, for example, by ensuring that users are easily finding what they are looking for.

Functionality Cookies:

These cookies are used to recognize you when you return to our website. This enables us to personalize our content for you, greet you by name and remember your preferences (for example, your choice of language or region). Loss of the information in these cookies may make our services less functional, but would not prevent the website from working.

Targeting/Profiling Cookies:

These cookies record your visit to our website and/or your use of the services, the pages you have visited and the links you have followed. We will use this information to make the website and the advertising displayed on it more relevant to your interests. We may also share this information with third parties for this purpose.

Analog Devices is a global leader in the design and manufacturing of analog, mixed signal, and DSP integrated circuits to help solve the toughest engineering challenges.

Interested in the latest news and articles about ADI products, design tools, training and events? Choose from one of our 12 newsletters that match your product area of interest, delivered monthly or quarterly to your inbox.